Aluminum and aluminum alloy components are easily mechanically processed (e.g., cut or polished), have excellent corrosion resistance, and improve fuel efficiency due to their light weight. In view or these aspects, they are widely used in automotive components, such as aluminum wheels. The aluminum wheel is coated with a primer coating composition; this coating film is required to have properties, such as corrosion resistance, edge corrosion resistance, and filiform corrosion resistance.
Recent aluminum wheels are increasingly being given complex designs to provide a luxurious appearance, resulting in a greater number of sharply cut edge portions. To protect the edge portions by coating, coating compositions with various innovations have been developed.
Patent Literature (PTL) 1, for example, discloses a primer coating composition comprising 100 parts by weight of self-crosslinkable acrylic resin, 10 to 20 parts by weight of ultrafine particle silica, 3 to 10 parts by weight of epoxy resin, and 5 to 20 parts by weight of a silane coupling agent.
Additionally, PTL 2 discloses an aluminum wheel coating method comprising applying a specific anticorrosive coating composition (printer coating composition) to an aluminum wheel, and subsequently applying a clear coating composition. This anticorrosive coating composition contains a specific acrylic resin and a specific epoxy resin, and optionally a silica powder.
However, aluminum, wheels are sometimes used in harsh corrosive environments where salt damage is caused while driving due to snow-melting salt spread in cold regions. Therefore, the coating films formed of the primer coating compositions disclosed in PTL 1 or 2 had insufficient performance in corrosion resistance, edge corrosion resistance, or filiform corrosion resistance.